As an exhaust gas NOx reduction system for reducing NOx in an exhaust gas from a diesel engine, an SCR system using a selective catalytic reduction device (hereinafter referred to as SCR (selective catalytic reduction) device) has been developed.
The SCR system supplies urea stored in a urea tank to an exhaust gas upstream of the SCR device to generate ammonia, and using the ammonia, reduces NOx on an SCR catalyst to reduce NOx (see, for example, patent literature 1).
In an SCR system, when there is a small or no remaining quantity of urea stored in a urea tank, the urea injection quantity is limited or urea injection is stopped. If a vehicle is run in such state, NOx in the exhaust gas is emitted to the atmosphere without being reduced. Therefore, in a vehicle with an SCR system installed therein, when there is a small or no remaining quantity of urea stored in an urea tank, control is performed to provide a speed limit or regulate (or prohibit) an engine start.
The remaining quantity of urea stored in the urea tank is detected by a urea level sensor provided in the urea tank. For the urea level sensor, a float-type level sensor that includes a float following a liquid surface of urea and detects a position of the float to measure a liquid level of urea in the urea tank is used.
In the float-type level sensor, if a float is immobilized due to any reason, the liquid level cannot be detected. Also, since a liquid level of urea (that is, a remaining quantity of urea), which is a value detected by the urea level sensor, is used for control to provide a speed limit and/or an engine start limit, the urea level sensor may be subjected to an action called tampering in which a float is intentionally fixed to make urea that does actually not remain, seem to remain in order to avoid these limits. Accordingly, there is a need for a diagnosis of whether or not a float is stuck to detect an abnormality (failure) of the level sensor if the float is immobilized or the float is intentionally fixed.
For conventional diagnoses of whether or not a float is stuck, for example, for fuel level sensors that detect a liquid level of a fuel in a fuel tank, a method in which a used quantity of fuel is calculated from a running distance and a fuel quantity obtained from a change in value detected by the fuel level sensor, and if there is a large difference between the fuel quantity obtained from the change in value detected by the fuel level sensor and the fuel quantity obtained from the running distance, it is determined that the fuel level sensor has an abnormality (that is, a float is stuck) is known (see, for example, patent literature 2).